1. Field of the Invention
This disclosure relates to reducing intraocular pressure within the eye. This disclosure also relates to a treatment of glaucoma and/or other ocular disorders wherein aqueous humor is permitted to flow out of an anterior chamber of the eye through a surgically implanted pathway.
2. Description of the Related Art
A human eye is a specialized sensory organ capable of light reception and is able to receive visual images. Aqueous humor is a transparent liquid that fills at least the region between the cornea, at the front of the eye, and the lens. A trabecular meshwork, located in an anterior chamber angle, which is formed between the iris and the cornea, normally serves as a drainage channel for aqueous humor from the anterior chamber so as to maintain a balanced pressure within the anterior chamber of the eye.
About two percent of people in the United States have glaucoma. Glaucoma is a group of eye diseases encompassing a broad spectrum of clinical presentations, etiologies, and treatment modalities. Glaucoma causes pathological changes in the optic nerve, visible on the optic disk, and it causes corresponding visual field loss, resulting in blindness if untreated. Lowering intraocular pressure is the major treatment goal in all glaucomas.
In glaucomas associated with an elevation in eye pressure (intraocular hypertension), the source of resistance to outflow is mainly in the trabecular meshwork. The tissue of the trabecular meshwork normally allows the aqueous humor (hereinafter referred to as “aqueous”) to enter Schlemm's canal, which then empties into aqueous collector channels in the posterior wall of Schlemm's canal and then into aqueous veins, which form the episcleral venous system. Aqueous is continuously secreted by a ciliary body around the lens, so there is a constant flow of aqueous from the ciliary body to the anterior chamber of the eye. Pressure within the eye is determined by a balance between the production of aqueous and its exit through the trabecular meshwork (major route) and uveoscleral outflow (minor route). The portion of the trabecular meshwork adjacent to Schlemm's canal (the juxtacanilicular meshwork) causes most of the resistance to aqueous outflow.
While a majority of the aqueous leaves the eye through the trabecular meshwork and Schlemm's canal, it is believed that about 10 to about 20 percent of the aqueous in humans leaves through the uveoscleral pathway. The degree with which uveoscleral outflow contributes to the total outflow of the eye appears to be species dependent. As used herein, the term “uveoscleral outflow pathway” is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the space or passageway whereby aqueous exits the eye by passing through the ciliary muscle bundles located angle of the anterior chamber and into the tissue planes between the choroid and the sclera, which extend posteriorly to the optic nerve. From these tissue planes, it is believed that the aqueous travels through the surrounding scleral tissue and drains via the scleral and conjunctival vessels, or is absorbed by the uveal blood vessels. It is unclear from studies whether the degree of physiologic uveoscleral outflow is pressure-dependent or pressure-independent. As used herein, the term “supraciliary space” is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the portion of the uveoscleral pathway through the ciliary muscle and between the ciliary body and the sclera, and the term “suprachoroidal space” is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the portion of the uveoscleral pathway between the choroid and sclera. Although it is not completely understood, some studies have suggested that there may be a “compact zone” of connective tissue associated with the junction between the retina and ciliary body, known as the ora serrata. This “compact zone” may act as a site of resistance along the uveoscleral outflow pathway. The ora serrata can vary in length from about 5.75 mm to 7.5 mm nasally to about 6.5 mm to about 8.5 mm temporally. Other studies suggest that the ciliary muscle bundles are the primary site of resistance.
Certain therapeutic agents have been shown to reduce intraocular pressure by increasing uveoscleral outflow, but the mechanism by which uveoscleral outflow is increased is unclear. Some studies have suggested that relaxation of the ciliary muscle may reduce resistance through the ciliary muscle bundles to increase flow. Other studies suggest that dilation of the post-capillary venules or constriction of the pre-capillary arterioles may reduce downstream fluid pressure and increase uveoscleral outflow.
Glaucoma is broadly classified into two categories: closed-angle glaucoma, also known as angle closure glaucoma, and open-angle glaucoma. Closed-angle glaucoma is caused by closure of the anterior chamber angle by contact between the iris and the inner surface of the trabecular meshwork. Closure of this anatomical angle prevents normal drainage of aqueous from the anterior chamber of the eye. Open-angle glaucoma is any glaucoma in which the exit of aqueous through the trabecular meshwork is diminished while the angle of the anterior chamber remains open. For most cases of open-angle glaucoma, the exact cause of diminished filtration is unknown. Primary open-angle glaucoma is the most common of the glaucomas, and is often asymptomatic in the early to moderately advanced stages of glaucoma. Patients may suffer substantial, irreversible vision loss prior to diagnosis and treatment. However, there are secondary open-angle glaucomas that may include edema or swelling of the trabecular spaces (e.g., from corticosteroid use), abnormal pigment dispersion, or diseases such as hyperthyroidism that produce vascular congestion.
All current therapies for glaucoma are directed toward decreasing intraocular pressure. Currently recognized categories of drug therapy for glaucoma include but are not limited to: (1) Miotics (e.g., pilocarpine, carbachol, and acetylcholinesterase inhibitors), (2) Sympathomimetics (e.g., epinephrine and dipivalylepinephxine), (3) Beta-blockers (e.g., betaxolol, levobunolol and timolol), (4) Carbonic anhydrase inhibitors (e.g., acetazolamide, methazolamide and ethoxzolamide), and (5) Prostaglandins (e.g., metabolite derivatives of arachindonic acid). Medical therapy includes topical ophthalmic drops or oral medications that reduce the production of aqueous or increase the outflow of aqueous. However, drug therapies for glaucoma are sometimes associated with significant side effects. The most frequent and perhaps most serious drawback to drug therapy, especially the elderly, is patient compliance. Patients often forget to take their medication at the appropriate times or else administer eye drops improperly, resulting in under- or overdosing. Patient compliance is particularly problematic with therapeutic agents requiring dosing frequencies of three times a day or more, such as pilocarpine. Because the effects of glaucoma are irreversible, when patients dose improperly, allowing ocular concentrations to drop below appropriate therapeutic levels, further permanent damage to vision occurs. Furthermore, current drug therapies are targeted to be deposited directly into the ciliary body where the aqueous is produced. And current therapies do not provide for a continuous slow-release of the drug. When drug therapy fails, surgical therapy is pursued.
Surgical therapy for open-angle glaucoma consists of laser trabeculoplasty, trabeculectomy, and implantation of aqueous shunts after failure of trabeculectomy or if trabeculectomy is unlikely to succeed. Trabeculectomy is a major surgery that is widely used and is augmented with topically applied anticancer drugs, such as 5-fluorouracil or mitomycin-C to decrease scarring and increase the likelihood of surgical success.
Approximately 100,000 trabeculectomies are performed on Medicare-age patients per year in the United States. This number would likely increase if ocular morbidity associated with trabeculectomy could be decreased. The current morbidity associated with trabeculectomy consists of failure (10-15%); infection (a life long risk of 2-5%); choroidal hemorrhage, a severe internal hemorrhage from low intraocular pressure, resulting in visual loss (1%); cataract formation; and hypotony maculopathy (potentially reversible visual loss from low intraocular pressure). For these reasons, surgeons have tried for decades to develop a workable surgery for reducing intraocular pressure.
The surgical techniques that have been tried and practiced are goniotomy/trabeculotomy and other mechanical disruptions of the trabecular meshwork, such as trabeculopuncture, goniophotoablation, laser trabecular ablation, and goniocurretage. These are all major operations and are briefly described below.
Goniotomy and trabeculotomy are simple and directed techniques of microsurgical dissection with mechanical disruption of the trabecular meshwork. These initially had early favorable responses in the treatment of open-angle glaucoma. However, long-term review of surgical results showed only limited success in adults. In retrospect, these procedures probably failed due to cellular repair and fibrosis mechanisms and a process of “filling in.” Filling in is a detrimental effect of collapsing and closing in of the created opening in the trabecular meshwork. Once the created openings close, the pressure builds back up and the surgery fails.
Q-switched Neodynium (Nd) YAG lasers also have been investigated as an optically invasive trabeculopuncture technique for creating full-thickness holes in trabecular meshwork. However, the relatively small hole created by this trabeculopuncture technique exhibits a filling-in effect and fails.
Goniophotoablation is disclosed by Berlin in U.S. Pat. No. 4,846,172 and involves the use of an excimer laser to treat glaucoma by ablating the trabecular meshwork. This method did not succeed in a clinical trial. Hill et al. used an Erbium YAG laser to create full-thickness holes through trabecular meshwork (Hill et al., Lasers in Surgery and Medicine 11:341346, 1991). This laser trabecular ablation technique was investigated in a primate model and a limited human clinical trial at the University of California, Irvine. Although ocular morbidity was zero in both trials, success rates did not warrant further human trials. Failure was again from filling in of surgically created defects in the trabecular meshwork by repair mechanisms. Neither of these is a viable surgical technique for the treatment of glaucoma.
Goniocurretage is an “ab interno” (from the inside), mechanically disruptive technique that uses an instrument similar to a cyclodialysis spatula with a microcurrette at the tip. Initial results were similar to trabeculotomy: it failed due to repair mechanisms and a process of filling in.
Although trabeculectomy is the most commonly performed filtering surgery, viscocanalostomy (VC) and nonpenetrating trabeculectomy (NPT) are two new variations of filtering surgery. These are “ab externo” (from the outside), major ocular procedures in which Schlemm's canal is surgically exposed by making a large and very deep scleral flap. In the VC procedure, Schlemm's canal is cannulated and viscoelastic substance injected (which dilates Schlemm's canal and the aqueous collector channels). In the NPT procedure, the inner wall of Schlemm's canal is stripped off after surgically exposing the canal.
Trabeculectomy, VC, and NPT involve the formation of an opening or hole under the conjunctiva and scleral flap into the anterior chamber, such that aqueous is drained onto the surface of the eye or into the tissues located within the lateral wall of the eye. These surgical operations are major procedures with significant ocular morbidity. When trabeculectomy, VC, and NPT are thought to have a low chance for success, a number of implantable drainage devices have been used to ensure that the desired filtration and outflow of aqueous through the surgical opening will continue. The risk of placing a glaucoma drainage device also includes hemorrhage, infection, and diplopia (double vision).
All of the above embodiments and variations thereof have numerous disadvantages and moderate success rates. They involve substantial trauma to the eye and require great surgical skill in creating a hole through the full thickness of the sclera into the subconjunctival space. The procedures are generally performed in an operating room and involve a prolonged recovery time for vision. The complications of existing filtration surgery have prompted ophthalmic surgeons to find other approaches to lowering intraocular pressure.
Because the trabecular meshwork and juxtacanilicular tissue together provide the majority of resistance to the outflow of aqueous, they are logical targets for surgical removal in the treatment of open-angle glaucoma. In addition, minimal amounts of tissue need be altered and existing physiologic outflow pathways can be utilized. Some procedures bypass the trabecular meshwork and juxtacanilicular tissue to drain fluid to physiologic outflow channels. However, in severe cases, it has been found that these procedures do not sufficiently reduce intraocular pressure.
As reported in Arch. Ophthalm. (2000) 118:412, glaucoma remains a leading cause of blindness, and filtration surgery remains an effective, important option in controlling glaucoma. However, modifying existing filtering surgery techniques in any profound way to increase their effectiveness appears to have reached a dead end.